1. Field of the Invention
The present invention relates to a synchronous control system for effecting a synchronous and superimposed operation among different channels of a multi-channel-control machine tool, and more specifically, to a synchronous control system wherein the reliability of the system is improved when the synchronous and superimposed operation is effected.
2. Description of the Related Art
Numerically controlled machine tools, and in particular numerically controlled lathes and the like, are provided with a multiplicity of tool posts to enable a workpiece to be cut by a plurality of tools, to thereby increase the cutting speed. Accordingly, some of the tool posts must effect not only a movement relative the movement of a spindle but also a movement including the amount of movement of the spindle. This operation is referred to as a superimposed operation.
FIG. 3 is a schematic block diagram of a numerically controlled lathe as a multi-channel-control machine tool provided with a plurality of tool posts, wherein 61 designates a spindle controlled in a Z direction, herein referred to as a Z1-axis. In the numerically controlled lathe, the Z-axis is operated synchronously with the rotation of the spindle 61.
In FIG. 3, 62 designates a first tool post controlled only in an X direction, herein referred to as an X1-axis. Note, the spindle 61 and the first tool post 62 constitute a usual X - Z coordinate.
Designated at 63 is a second tool post controlled in the X- and Z-axis directions, herein referred to as an X2-axis and a Z2-axis. The second tool post 63 is independently operated on the X - Z coordinate when the spindle 61 does not move along the Z1-axis. However, when the spindle 61 is moved in the Z1-axis direction, the operation of the Z2-axis is superimposed with that of the Z1-axis of the spindle 61, since the movement of the Z2-axis must include the amount of movement of the Z1- axis. More specifically, the Z2-axis must be moved by an amount determined by adding the amount of movement of the Z1-axis to the original amount of movement of the Z2-axis.
Designated at 64 is a third tool post controlled only in the Z-axis direction, herein referred to as a Z3-axis. The operation of the Z3-axis is also superimposed with that of the Z1-axis.
When the operation of the multi-channel-control machine tool is superimposed as described above, each coordinate axis is controlled by a respective servomotor, but since a conventional servomotor control system can not detect a relative follow-up error therebetween, an alarm condition cannot be detected even if the control of the servomotors is erroneously effected.
More specifically, although servomotor control circuits for controlling the Z1-axis and the Z2-axis in FIG. 3 independently monitor an excessive follow-up error wherein a follow-up error amount exceeds a predetermined amount, they can not monitor a relative follow-up error amount or the like.